How cheap do you want? A couple hundred dollars is what they cost, but you can't use the results in court. Although I did hear of one man taking the cheap test results to the child support agency and getting his child support obligation cut.
They don't even need blood to work, just scrape some cells from inside the cheeks (mouth cheeks) onto a little swab and send them to be tested.
You can even use hair you pulled from a comb or brush, but I think those are more expensive.
But if you do it, and the test comes out negative, and your wife ever finds out about it, you're mud.
Fortunately, I have no doubt. Both my daughter and son are dead ringers for me, and that's scary!